The ultimate goal of this project is the elucidation of the role of the thyroid hormones in the maturation and development of the brain and the influence of the hormones in the regulation of cerebral metabolic processes. A necessary intermediate objective is the determination of the molecular mechanisms of the physiological and biochemical actions of the thyroid hormones. Earlier work done previously in this project demonstrated a stimulating action of thyroid hormones on protein biosynthesis. The mechanism of action of thyroxine still remains the primary focus of the project. The effects of thyroid hormones on radioactive amino acid incorporation into protein are examined in cell-free protein synthesizing systems from various mammalian tissues, such as liver, brain, reticulocytes, etc. Studies are occasionally carried out in vivo, in which the uptake of amino acids into protein in various tissues is measured in the intact animal or the effects of experimentally induced thyroid disease on various metabolic activities are examined. A wide variety of enzyme purification and assay procedures are also employed. Enzymes are purified from brain or liver homogenates by detergent treatment, salt fractionation, gel adsorption and filtration, ion exchange chromatography, etc. Assays of enzyme activities utilize chemical analyses, spectrophotometric techniques, and Geiger-Mueller and liquid scintillation radioactivity measurements.